1. Field of the Invention
The present invention relates to a sewing machine operable to form a buttonhole stitch.
2. Background Art
When forming a buttonhole stitch using a sewing machine, it is necessary to change a stitching length in accordance with a size of a button. Thus, a buttonhole stitch device operable to change the stitching length in accordance with the size of a button is employed in the sewing machine (see, e.g., Japanese Patent No. 3151923).
As shown in FIG. 9, a buttonhole stitch device 100 includes a presser member 101 attached to a lower end of a presser bar of a sewing machine. A presser frame 102 is disposed on the presser member 101 so as be slidable along a cloth feeding direction. A hollow stopper 102a is fixed to an upper surface of an end portion of the presser frame 102 on a side of a direction A, and a spiral spring 103 is disposed inside the stopper 102a. As shown in FIG. 9, the presser member 101 and stopper 102a are constantly biased toward each other by an elastic force of the spiral spring 103 such that they are in contact with each other at a buttonhole stitch start position.
On the other hand, a button holding device 104 is provided on the other end portion of the presser frame 102 on a side of a direction B, and the button holding device 104 is operable to hold a button B. The button holding device 104 includes a holding portion 104a which is slidable along the cloth feeding direction in accordance with a size of a button to be held.
A potentiometer 105 extending on an outer side of the presser frame 102 along the cloth feeding direction is attached to the presser member 101, and a detecting portion 106 which contacts the potentiometer 105 is attached to the holding portion 104a. The potentiometer 105 includes a variable resistor, and as shown in FIG. 10, when the presser frame 102 moves so that a contacting position of the detecting portion 105 and the potentiometer 105 is varied, a resistance value of the variable resistor varies. According to a value of a voltage to be applied to the thus varying resistance, an amount of a displacement of the presser frame 102 (i.e., the stitching length) can be detected.
At the time when the buttonhole stitch is started, an initial value of a voltage applied to the potentiometer 105 is stored in a memory of a control device. The control device starts to drive a sewing machine motor and a needle oscillation motor so as to form a bar tack 110a as shown in FIG. 11. Then, the control device changes a needle oscillation amount and drives a cloth feeding motor so as to stitch a left side portion 110b of a buttonhole. During a sewing operation, the control device checks whether a resistance value of the potentiometer 105 is 0 or not. When the control device judges that the resistance value became 0, the control device drives the sewing machine motor and the needle oscillation motor so as to form a bar tack 110c. Then, the control device again changes the needle oscillation amount and drives the cloth feeding motor so as to stitch right side portion 110d of the buttonhole. In the meantime, the control device again checks whether the resistance value of the potentiometer 105 is 0 or not, and when the control device judges that the resistance value became 0, the control device ends the sewing operation.
However, in such a buttonhole stitch device 100, because the potentiometer 105 mounted on the presser member 101 is large and long relative to the pressing member 101, the potentiometer 105 is likely to become rickety and tilted. Thus, a detection error is easy to occur when detecting the position of the presser frame 102. Moreover, because the presser frame 102 slidingly moves, an arm member coupling the detecting portion 106 and the holding portion 104a needs to be relatively long. Thus, the arm member is likely to become rickety and flexed, so that an error in a timing, at which the position where the resistance value becomes 0 is detected, is easy to occur. Accordingly, it has been difficult to enhance quality of the buttonhole stitch.